RESISTANCE TO THE SULFONYLUREA HERBICIDES CHLORSULFURON, AMIDOSULFURON, AND DPX-R9674 IN TRANSGENIC FLUE-CURED TOBACCO

Only one herbicide is currently available for preemergence broadleaf weed control in flue-cured tobacco (Nicotiana tabacum L.) grown in Canada. The high cost of registration, coupled with the small crop size, has resulted in few new products becoming available. Herbicide resistance introduced into tobacco by Agrobacterium-mediated transformation may allow the use of products with existing or impending registrations. We used two new, low-residual sulfonylurea herbicides: amidosulfuron (3-(4,6-dimethoxyprymidin-2-yl)-1-(N-methyl-N-methylsulfonyl-aminosulfonylurea) and DPX-R9674, which is a mixture of thifensulfuron (methyl-3-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl] amino] sulfonyl]-2-thiophenecarboxylate) and tribenuron methyl 2[[[[N-4-methoxy-6-methyl-1,3,5-triazin-2-yl)methylamino] carbonyl] amino] sulfonyl] benzoate). These two and chlorsulfuron (2-chloro-N-[(4-methoxy-6-methyl-1,3,5-triazin-2-yl) aminocarbonyl] benzenesulfonamide) were applied to two transgenic tobacco genotypes harboring the csr1-1 gene for chlorsulfuron resistance and compared with an untransformed control. Our purpose was to determine if transgenic seedlings were resistant to DPX-R9674 and amidosulfuron. The experiment was a factorial in a completely randomized design with 25 replications. The three herbicides were applied to the transgenic and control seedlings at three rates. The transgenic seedlings had significantly higher leaf area, top dry weight, and root dry weight than the untransformed control when sprayed with any of the three herbicides. Seedlings were highly resistant to amidosulfuron and chlorsulfuron. Resistance to DPX-R9674 in the transgenic seedlings was minimal, which was unexpected, considering that an analysis of AHAS activity revealed high levels of cross-resistance to chlorsulfuron, DPX-R9674, and amidosulfuron. It is possible that DPX-R9674 is metabolized into products that are herbicidally active at different AHAS binding sites. One of the transgenic lines was more resistant to herbicide application than the other indicating that selection for maximum gene expression among transgenic lines is a necessary part of transgenic cultivar development. It was concluded that DPX-R9674 would not be suitable for use with transgenic crops harboring the csr1-1 gene for chlorsulfuron resistance. The other low-residual sulfonylurea, amidosulfuron, was more promising.